1. Field of the Invention
This invention relates generally to telephone signals, and, more particularly, to detecting a "message waiting" signal.
2. Description of the Related Art
There is a current trend among telecommunications providers to offer specialized services to users for greater convenience, flexibility, and reliability. The specialized services may include Caller ID, Call Blocker, Voice Message Recording (VMR), or the like. One specific example of the Voice Message Recording is Southwestern Bell's CallNotes.RTM..
CallNotes.RTM. and other similar VMR services offer many advantages over conventional answering machines. For instance, CallNotes.RTM. allows subscribers to receive messages even while the telephone line is in use, either for voice and/or data communication. Additionally, VMR services offer many of the features of a conventional answering machine, without many of the accompanying restrictions. Because the messages are stored at a central location determined by the service provider, a subscriber may easily check messages from remote locations even when the subscriber's phone line is in use.
A subscriber having CallNotes.RTM. or its equivalent service can check messages by lifting a telephone and detecting a distinct "stutter" on the dial tone. Generally, the "stutter" tone lasts for a preselected interval before the standard, continuous tone resumes. Upon detection of the "stutter" tone, the subscriber can call a predetermined telephone number to retrieve messages. As mentioned above, the messages can also be retrieved from a remote location by simply calling the same telephone number. While the "stutter" tone achieves its intended purpose, customer demand has prompted service providers to seek a more visual, and less hands-on, method of detecting messages left by callers.
Service providers responded to customer demand by offering its subscribers telephones or stand-alone devices equipped with a Visual Message Waiting Indicator (VMWI). A VMWI comprises a light emitting diode (LED) that turns on or blinks periodically after a message is detected. The VMWI is activated upon detection of a Frequency Shift Keying (FSK) signal, a signal that is generated by service providers any time a caller leaves a message. Typically, the FSK signal is only sent once to a subscriber's telephone to indicate the presence of a message.
It is possible that in some instances a telephone device equipped with the VMWI may not be able to detect the FSK signal. This is particularly true if a condition exists where the telephone is in use but not connected to an external telephone line, where the external telephone line may be a Public Switched Telephone Network (PSTN) line or a Private Branch Exchange (PBX) line. An example of such a condition can be seen in a cordless telephone having a local conference call feature (e.g., communications between a handset and a base station). For illustrative purposes, it is assumed that the cordless telephone is connected to a PSTN line.
During a local conference call, the PSTN line is isolated from the analog-to-digital (A/D) and digital-to-analog (D/A) circuits of the cordless telephone. The A/D and D/A circuits are utilized for communication between the handset and base station for the duration of the local conference call. The isolation of the PSTN line from the A/D circuit is necessary to prevent the dial tone from being heard during the conference call. On the other hand, to detect the FSK signal, the PSTN line must be connected to the A/D circuit at all times. Thus, if the PSTN line is isolated from the A/D path during a local conference call, it is not possible to detect an incoming FSK signal for the duration of the conference call. An FSK signal received during a local conference call goes undetected even after the termination of that call. Hence, without the detection of the FSK signal, the VMWI will remain off, leaving the subscriber unaware of the pending message.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.